The present invention relates to a phenol resin composition for forming pulleys used in motor vehicles, being capable of forming a resin pulley which is free from swelling or shell cracking on the surface at a high temperature of 250xc2x0 C. or more and exhibits high mechanical strength and superior thermal shock resistance, and to a phenol resin pulley for motor vehicles formed from the resin.
A phenol resin pulley is light and noiseless as compared with metal pulleys and, as compared with resin pulleys made from thermoplastic resins such as nylon and polyphenyl sulfide, the phenol resin pulley excels in dimensional accuracy and is free from melting or deformation when the pulley is overheated in emergencies or the like. Therefore, the phenol resin pulley is suitably used for motor vehicles requiring lightness, quietness, high reliability, and the like. Conventionally, phenol resin pulleys using organic fiber such as cellulose as a main substrate have been widely used to satisfy the light weight and belt strength properties required for such pulleys.
A phenol resin pulley using organic fiber as a main substrate, however, sometimes swells or cracks on the surface due to decomposition of the organic fiber when the pulley is instantaneously overheated at 250xc2x0 C. or above due to belt slip. This may cause chips in the V-groove portion of the pulley and accelerate deterioration of the belt.
As a phenol resin pulley which suppresses such swelling and shell cracking, the phenol resin pulley using aramid fiber having thermal resistance higher than cellulose as organic fiber has been known. However, the aramid fiber is not only extremely expensive, but also is very difficult to disperse in phenol resins and the like, giving rise to poor productivity when producing a resin composition by kneading with phenol resins and the like. In addition, the resulting resin composition exhibits only poor formability. For these reasons, the aramid fiber is difficult to employ as a general-purpose material.
One phenol resin forming material for pulleys employs a phenol aralkyl resin as a phenol resin component. A phenol aralkyl resin is a novolak-type resin using hexamethylenetetramine as a curing agent. Therefore, unreacted hexamethylenetetramine may remain in phenol resin pulleys. Because hexamethylenetetramine vaporizes at a temperature of 200xc2x0 C. or below, the surface of a pulley may swell if the temperature increases above 250xc2x0 C. as mentioned above.
In addition to the non-swelling and shell crack-free properties, a phenol resin pulley must have high mechanical strength. As a phenol resin pulley exhibiting improved mechanical strength, a phenol resin pulley with glass fiber incorporated therein has conventionally been known. In spite of the improvement in mechanical strength and a decrease in the coefficient of linear expansion, this pulley exhibits an increased modulus of elasticity and decreased tensile elongation, rendering the pulley easily broken by thermal shock.
Japanese Patent Application Laid-open No. 217818/1997 discloses a phenol resin pulley formed from a resin composition containing a phenol resin, inorganic fiber, inorganic powder, organic fiber, and elastomer. This pulley play is claimed to exhibit excellent wear resistance and the heat shock resistance.
However, this pulley begins to decompose at a temperature lower than 250xc2x0 C. because the pulley contains organic materials such as organic fiber and elastomer, in addition to glass fiber which is an inorganic material. For this reason, the pulley swells, produces shell cracks, or exhibits a decrease in the mechanical strength under conditions of 250xc2x0 C. or higher.
An object of the present invention, therefore, is to provide a phenol resin forming material for a pulley used in motor vehicles which is free from swelling or shell cracking on the surface at a high temperature of 250xc2x0 C. or more and exhibits high mechanical strength and superior thermal shock resistance.
In view of the above-described situation, the inventor of the present invention has conducted extensive studies and, as a result, has found that a phenol resin composition comprising resol, glass fiber, specific inorganic powder, and polyvinyl butyral in a specific ratio can form a pulley for motor vehicles which is almost free from swelling or shell cracking on the surface at a high temperature of 250xc2x0 C. or more and exhibits high mechanical strength and superior thermal shock resistance. This finding has led to the completion of the present invention.
Specifically, the present invention is to provide a phenol resin composition for forming pulleys used in motor vehicles, comprising 35 to 45 wt % of resol, 35 to 45 wt % of glass fiber, 5 to 15 wt % of one or more inorganic powders selected from the group consisting of calcium carbonate, clay, and wallastonite, and 1 to 3 wt % of polyvinyl butyral.
The present invention also provides a pulley for motor vehicles formed from the phenol resin composition.